ONEOK INC /NEW/
8-K, 1999-02-08
NATURAL GAS TRANSMISISON & DISTRIBUTION
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                       SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C.  20549



                                    FORM 8-K


                                 CURRENT REPORT
                     PURSUANT TO SECTION 13 OR 15(D) OF THE
                        SECURITIES EXCHANGE ACT OF 1934


                                February 8, 1999
                Date of Report (Date of earliest event reported)



                                  ONEOK, Inc.
             (Exact name of registrant as specified in its charter)


         Oklahoma                        1-2572                   73-1520922
(State or other jurisdiction          (Commission               (IRS Employer
     of incorporation)                File Number)           Identification No.)

                        100 West Fifth Street, Tulsa, OK
                    (Address of principal executive offices)

                                     74103
                                   (Zip code)

                                 (918) 588-7000
              (Registrant's telephone number, including area code)

                                 Not Applicable
         (Former name or former address, if changed since last report)
<PAGE>
 
Item 1.  Not Applicable.

Item 5.  Other Events.

     On February 8, 1999, ONEOK, Inc. (the "Company") consummated an
underwritten public offering of $100,000,000 aggregate principal amount of the
Company's 6% Debentures Due February 1, 2009.  The Offering was made pursuant to
a Prospectus Supplement dated February 3, 1999 that supplements the Company's
Prospectus dated September 4, 1998 and that relates to the Company's
Registration Statement on Form S-3 (SEC File No. 333-62279).

     Additional information with respect to the Offering described herein is set
forth in the exhibits hereto.

Item 6.  Not Applicable.

Item 7.  Financial Statements, Pro Forma Financial Information and Exhibits.

Exhibit No.      Description

   (1)(a)        Underwriting Agreement.

   (1)(b)        Price Determination Agreement.

    (4)          Third Supplemental Indenture.


Item 8.  Not Applicable.

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<PAGE>
 
                                   SIGNATURES


     Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned hereunto duly authorized, on this 8th of February, 1999.

                                    ONEOK, Inc.



                                    By: /s/ Barry D. Epperson
                                        ________________________________________
                                        Vice President, Controller
                                        and Chief Accounting Officer

                                       3
<PAGE>
 
                                 EXHIBIT INDEX


Exhibit No.      Description

   (1)(a)        Underwriting Agreement.

   (1)(b)        Price Determination Agreement.

    (4)          Third Supplemental Indenture.

<PAGE>
 
                                                                 Exhibit (1)(a)


                                  ONEOK, INC.

                                  $100,000,000
                       6% Debentures Due February 1, 2009

                             UNDERWRITING AGREEMENT
                             ----------------------



                                                                February 3, 1999



PAINEWEBBER INCORPORATED
1285 Avenue of the Americas
New York, New York  10019

Dear Sirs:

     ONEOK, Inc., an Oklahoma corporation (the "Company"), proposes to issue and
sell an aggregate of $100,000,000 principal amount of the Company's 6%
Debentures Due February 1, 2009 (the "Securities") to be issued pursuant to an
Indenture, dated as of September 24, 1998, as amended or supplemented (the
"Indenture"), between the Company and Chase Bank of Texas, National Association,
as Trustee (the "Trustee"). The Securities will be sold to PaineWebber
Incorporated (the "Underwriter" or "you").

     The purchase price for the Securities to be paid by you shall be agreed
upon by the Company and the Underwriter, and such agreement shall be set forth
in a separate written instrument substantially in the form of Exhibit A hereto
(the "Price Determination Agreement"). The Price Determination Agreement may
take the form of an exchange of any standard form of written telecommunication
between the Company and the Underwriter and shall specify such applicable
information as is indicated in Exhibit A hereto. The offering of the Securities
will be governed by this Agreement, as supplemented by the Price Determination
Agreement. From and after the date of the execution and delivery of the Price
Determination Agreement, this Agreement shall be deemed to incorporate, and,
unless the context otherwise indicates, all references contained herein to "this
Agreement" and to the phrase "herein" shall be deemed to include the Price
Determination Agreement.

     The Company confirms as follows its agreements with the Underwriter.

     1.   Agreement to Sell and Purchase. On the basis of the representations,
warranties and agreements of the Company herein contained and subject to all the
terms and conditions of this Agreement, the Company agrees to sell to the
Underwriter, and the Underwriter agrees to purchase from the Company, the
principal amount of the Securities, all at the purchase price plus accrued
interest, if any, from February 1, 1999, to the Closing Date (as hereinafter
defined), to be
<PAGE>
 
agreed upon by the Underwriter and the Company and set forth in the Price
Determination Agreement.

     2.   Delivery and Payment. Delivery of the Securities shall be made to the
Underwriter against payment of the purchase price by wire transfer in
immediately available funds to an account specified by the Company. Such payment
shall be made at 10:00 a.m., New York City time, on the third business day after
the date on which the first bona fide offering of the Securities to the public
is made by you or at such time on such other date, not later than 10 business
days after such date as may be agreed upon by the Company and you (such date is
hereinafter referred to as the "Closing Date"). Delivery of the Securities shall
be made through the facilities of the Depository Trust Company unless you shall
otherwise instruct.

     3.   Representations and Warranties of the Company. The Company represents,
warrants and covenants to the Underwriter that:

          (a) The Company meets the requirements for use of Form S-3 and a
registration statement (Registration No. 333-62279) on Form S-3 relating to the
Securities including a preliminary prospectus and such amendments to such
registration statement as may have been required to the date of this Agreement,
has been prepared by the Company under the provisions of the Securities Act of
1933, as amended (the "Act"), and the rules and regulations (collectively
referred to as the "Rules and Regulations") of the Securities and Exchange
Commission (the "Commission") thereunder, and has been filed with the
Commission. The term "preliminary prospectus" as used herein means a preliminary
prospectus as contemplated by Rule 430 or Rule 430A ("Rule 430A") of the Rules
and Regulations included at any time as part of the registration statement.
Copies of such registration statement and amendments and of each related
preliminary prospectus have been delivered to you. The term "Registration
Statement" means the registration statement as amended at the time it became
effective (the "Effective Date"), including financial statements and all
exhibits and any information deemed to be included by Rule 430A or Rule 434 of
the Rules and Regulations. If the Company files a registration statement to
register a portion of the Securities and relies on Rule 462(b) of the Rules and
Regulations for such registration statement to become effective upon filing with
the Commission (the "Rule 462 Registration Statement"), then any reference to
the "Registration Statement" shall be deemed to include the Rule 462
Registration Statement, as amended from time to time. The term "Prospectus"
means the prospectus as first filed with the Commission pursuant to Rule 424(b)
of the Rules and Regulations in connection with the offering of the Securities
and the form of final prospectus included in the Registration Statement at the
Effective Date. Any reference herein to the Registration Statement, any
preliminary prospectus or the Prospectus shall be deemed to refer to and include
the documents incorporated by reference therein pursuant to Item 12 of Form S-3
which were filed under the Securities Exchange Act of 1934, as amended (the
"Exchange Act"), on or before the Effective Date or the date of such preliminary
prospectus or the Prospectus, as the case may be. Any reference herein to the
terms "amend," "amendment" or "supplement" with respect to the Registration
Statement, any preliminary prospectus or the Prospectus shall be deemed to refer
to and include the filing of any document under the Exchange Act after the
Effective Date, or the date of any preliminary prospectus or the Prospectus, as
the case may be, and deemed to be incorporated therein by reference.

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<PAGE>
 
          (b) On the Effective Date, the date the Prospectus is first filed with
the Commission pursuant to Rule 424(b) (if required), at all times subsequent to
and including the Closing Date and when any post-effective amendment to the
Registration Statement becomes effective or any amendment or supplement to the
Prospectus is filed with the Commission, the Registration Statement and the
Prospectus (as amended or as supplemented if the Company shall have filed with
the Commission any amendment or supplement thereto), including the financial
statements included or incorporated by reference in the Prospectus, did or will
comply with the applicable provisions of the Act, the Exchange Act, the rules
and regulations thereunder (the "Exchange Act Rules and Regulations"), the Trust
Indenture Act of 1939, as amended (the "Trust Indenture Act"), the rules and
regulations thereunder (the "Trust Indenture Act Rules and Regulations") and the
Rules and Regulations and will contain all statements required to be stated
therein in accordance with the Act, the Exchange Act, the Exchange Act Rules and
Regulations and the Rules and Regulations. On the Effective Date and when any
post-effective amendment to the Registration Statement becomes effective, no
part of the Registration Statement, or any such amendment did or will contain
any untrue statement of a material fact or omit to state a material fact
necessary to make the statements therein not misleading. At the Effective Date,
the date the Prospectus or any amendment or supplement to the Prospectus is
filed with the Commission and at the Closing Date, the Prospectus did not or
will not contain any untrue statement of a material fact or omit to state a
material fact necessary to make the statements therein, in the light of the
circumstances under which they were made, not misleading. The foregoing
representations and warranties in this Section 3(b) do not apply to any
statements or omissions made in reliance on and in conformity with information
relating to the Underwriter furnished in writing to the Company by you
specifically for inclusion in the Registration Statement or Prospectus or any
amendment or supplement thereto. For all purposes of this Agreement, the amounts
of the selling concession and reallowance set forth in the Prospectus under the
caption "Underwriting" constitute the only information relating to the
Underwriter furnished in writing to the Company by you specifically for
inclusion in the Registration Statement or the Prospectus. The Company has not
distributed any offering material in connection with the offering or sale of the
Securities other than the Registration Statement, the preliminary prospectus,
the Prospectus or any other materials, if any, permitted by the Act. On the
Effective Date, the date the Prospectus is first filed with the Commission
pursuant to Rule 497 (if required), and at all subsequent times to and including
the Closing Date, the Indenture did or will comply with all applicable
provisions of the Trust Indenture Act and the Trust Indenture Act Rules and
Regulations.

          (c) The documents which are incorporated by reference in the
Prospectus or from which information is so incorporated by reference, when they
become effective or were filed with the Commission, as the case may be, complied
in all material respects with the requirements of the Act or the Exchange Act,
as applicable, the Exchange Act Rules and Regulations and the Rules and
Regulations; and any documents so filed and incorporated by reference subsequent
to the date hereof shall, when they are filed with the Commission, conform in
all material respects with the requirements of the Act and the Exchange Act, as
applicable, the Exchange Act Rules and Regulations and the Rules and
Regulations.

          (d) The only subsidiaries (as defined in the Rules and Regulations) of
the Company are the subsidiaries listed on Exhibit 21 to the Company's Annual
Report on Form 10-K for the fiscal year ended August 31, 1998 (the
Subsidiaries"). The Company and each of its Subsidiaries is, and at the Closing
Date will be, a corporation duly organized, validly

                                       3
<PAGE>
 
existing and in good standing under the laws of its jurisdiction of
incorporation. The Company and each of its Subsidiaries has, and at the Closing
Date will have, full power and authority to conduct all the activities conducted
by it, to own or lease all the assets owned or leased by it and to conduct its
business as described in the Registration Statement and the Prospectus. The
Company and each of its Subsidiaries is, and at the Closing Date will be, duly
licensed or qualified to do business and in good standing as a foreign
corporation in all jurisdictions in which the nature of the activities conducted
by it or the character of the assets owned or leased by it makes such licensing
or qualification necessary, except to the extent that the failure to so qualify
or be in good standing would not have a material adverse effect on the condition
(financial or otherwise), earnings, business affairs or business prospects of
the Company and its Subsidiaries, considered as one enterprise (a "Material
Adverse Effect"). All of the outstanding shares of capital stock or capital
interests of the Subsidiaries have been duly authorized and validly issued and
are fully paid and non-assessable and are owned by the Company free and clear of
all liens, encumbrances and claims whatsoever. Except for the stock of the
Subsidiaries and as disclosed in the Registration Statement, the Company does
not own, and at the Closing Date will not own, directly or indirectly, any
shares of stock or any other equity or long-term debt securities of any
corporation or have any equity interest in any firm, partnership, joint venture,
association or other entity. Complete and correct copies of the certificate of
incorporation and of the by-laws of the Company and each of its Subsidiaries and
all amendments thereto have been delivered to you, and no changes therein will
be made subsequent to the date hereof and prior to the Closing Date.

          (e) The Securities have been duly and validly authorized and, when
authenticated by the Trustee and issued, delivered and sold in accordance with
this Agreement and the Indenture, will have been duly and validly executed,
authenticated, issued and delivered and will constitute valid and binding
obligations of the Company, enforceable against the Company in accordance with
their respective terms and entitled to the benefits provided by the Indenture.
At the Closing Date, the Company will have an authorized and outstanding
capitalization as set forth in the Prospectus.

          (f) The description of the Securities in the Registration Statement
and the Prospectus is, and at the Closing Date will be, complete and accurate in
all respects. The Indenture conforms to the description thereof contained in the
Registration Statement and the Prospectus.

          (g) The financial statements and schedules included or incorporated by
reference in the Registration Statement or the Prospectus present fairly the
consolidated financial condition of the Company or Southwest Gas Corporation, a
California corporation ("Southwest Gas"), as the case may be, as of the
respective dates thereof and the consolidated results of operations and cash
flows of the Company or Southwest Gas, as the case may be, for the respective
periods covered thereby, all in conformity with generally accepted accounting
principles applied on a consistent basis throughout the entire period involved,
except as otherwise disclosed in the Prospectus. The pro forma financial
statements and other pro forma financial information included in the
Registration Statement or the Prospectus (i) present fairly in all material
respects the information shown therein, (ii) have been prepared in accordance
with the Commission's rules and guidelines with respect to pro forma financial
statements and (iii) have been properly computed on the bases described therein.
The assumptions used in the preparation of the pro forma financial statements
and other pro forma financial information

                                       4
<PAGE>
 
included in the Registration Statement or the Prospectus are reasonable and the
adjustments used therein are appropriate to give effect to the transactions or
circumstances referred to therein. No other financial statements or schedules of
the Company or Southwest Gas are required by the Act, the Exchange Act or the
Rules and Regulations to be included in the Registration Statement or the
Prospectus. KPMG LLP and Arthur Andersen LLP (the "Accountants"), who have
reported on such financial statements and schedules, are independent accountants
with respect to the Company and Southwest Gas, respectively, as required by the
Act and the Rules and Regulations. There are no statements required to be
included in the Registration Statement or the Prospectus with respect to the
Accountants pursuant to Rule 509 of Regulation S-K of the Rules and Regulations.

          (h) The Company maintains a system of internal accountings control
sufficient to provide reasonable assurance that (i) transactions are executed in
accordance with management's general or specific authorization; (ii)
transactions are recorded as necessary to permit preparation of financial
statements in conformity with generally accepted accounting principles and to
maintain accountability for assets; (iii) access to assets is permitted only in
accordance with management's general or specific authorization; and (iv) the
recorded accountability for assets is compared with existing assets at
reasonable intervals and appropriate action is taken with respect to any
differences.

          (i) Subsequent to the respective dates as of which information is
given in the Registration Statement and the Prospectus and prior to the Closing
Date, except as set forth in or contemplated by the Registration Statement and
the Prospectus, (i) there has not been and will not have been any change in the
capitalization of the Company, or in the business, properties, business
prospects, condition (financial or otherwise) or results of operations of the
Company and its Subsidiaries, arising for any reason whatsoever, (ii) neither
the Company nor any of its Subsidiaries has incurred nor will it incur any
material liabilities or obligations, direct or contingent, nor has it entered
into nor will it enter into any material transactions other than pursuant to
this Agreement and the transactions referred to herein and (iii) the Company has
not and will not have paid or declared any dividends or other distributions of
any kind on any class of its capital stock, other than the ordinary quarterly
dividend paid or payable by the Company to holders of its common stock and
preferred stock consistent with past practices.

          (j) The Company is not an "investment company" or an "affiliated
person" of, or "promoter" or "principal underwriter" for, an "investment
company," as such terms are defined in the Investment Company Act of 1940, as
amended.

          (k) Except as set forth in the Registration Statement and the
Prospectus, there are no actions, suits or proceedings pending or threatened
against or affecting the Company or any of its Subsidiaries or any of their
respective directors or officers in their capacity as such, before or by any
Federal or state court, commission, regulatory body, administrative agency or
other governmental body, domestic or foreign, wherein an unfavorable ruling,
decision or finding might have a Material Adverse Effect.

          (l) The Company and each of its Subsidiaries has, and at the Closing
Date will have, (i) all governmental licenses, permits, consents, orders,
approvals and other authorizations (collectively, "Governmental Licenses")
necessary to carry on its business as contemplated in the Prospectus, except for
the Governmental Licenses the absence of which

                                       5
<PAGE>
 
would not have a Material Adverse Effect, (ii) complied in all material respects
with all laws, regulations and orders applicable to it or its business and (iii)
performed all its material obligations required to be performed by it, and is
not, and at the Closing Date will not be, in default, under any indenture,
mortgage, deed of trust, voting trust agreement, loan agreement, bond,
debenture, note agreement, lease, contract or other agreement or instrument
(collectively, a "contract or other agreement") to which it is a party or by
which its property is bound or affected, other than defaults that would not,
individually or in the aggregate, result in a Material Adverse Effect. To the
best knowledge of the Company and each of its Subsidiaries, no other party under
any contract or other agreement to which it is a party is in material default in
any respect thereunder. Neither the Company nor any of its Subsidiaries is, nor
at the Closing Date will any of them be, in violation of any provision of its
certificate of incorporation or by-laws.

          (m) No consent, approval, authorization or order of, or any filing or
declaration with, any court or governmental agency or body is required in
connection with the authorization, issuance, transfer, sale or delivery of the
Securities by the Company, in connection with the execution, delivery and
performance of this Agreement by the Company or in connection with the taking by
the Company of any action contemplated hereby and in the Indenture and the
Securities, except such as have been obtained under the Act, the Trust Indenture
Act, the Trust Indenture Act Rules and Regulations or the Rules and Regulations
and such as may be required under state securities or Blue Sky laws or the by-
laws and rules of the National Association of Securities Dealers, Inc. (the
"NASD") in connection with the purchase and distribution by the Underwriter of
the Securities.

          (n) The Company has full corporate power and authority to enter into
this Agreement. This Agreement has been duly authorized, executed and delivered
by the Company and constitutes a valid and binding agreement of the Company and
is enforceable against the Company in accordance with the terms hereof. The
Indenture has been duly authorized and constitutes a valid and binding agreement
of the Company and is enforceable against the Company in accordance with its
terms. The performance by the Company of this Agreement, the Indenture and the
Securities and the consummation of the transactions contemplated hereby and
thereby and the application of the net proceeds from the offering and sale of
the Securities to be sold by the Company in the manner set forth in the
Prospectus under "Use of Proceeds" will not result in the creation or imposition
of any lien, charge or encumbrance upon any of the assets of the Company or any
of its Subsidiaries pursuant to the terms or provisions of, or result in a
breach or violation of any of the terms or provisions of, or constitute a
default under, or give any other party a right to terminate any of its
obligations under, or result in the acceleration of any obligation under, the
certificate of incorporation or by-laws of the Company or any of its
Subsidiaries, any material contract or other agreement to which the Company or
any of its Subsidiaries is a party or by which the Company or any of its
Subsidiaries or any of its properties is bound or affected, or violate or
conflict with any judgment, ruling, decree, order, statute, rule or regulation
of any court or governmental agency or body applicable to the business or
properties of the Company or any of its Subsidiaries.

          (o) The Company and each of its Subsidiaries has good and marketable
title to all properties and assets described in the Prospectus as owned by it,
free and clear of all liens, charges, encumbrances or restrictions, except such
as are described in the Prospectus or are not material to the business of the
Company or its Subsidiaries. The Company and each of its Subsidiaries has valid,
subsisting and enforceable leases for the properties described in the

                                       6
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Prospectus as leased by it, with such exceptions as are not material and do not
materially interfere with the use made and proposed to be made of such
properties by the Company and such Subsidiaries.

          (p) There is no document or contract of a character required to be
described in the Registration Statement or the Prospectus or to be filed as an
exhibit to the Registration Statement which is not described or filed as
required. All such contracts to which the Company or any Subsidiary is a party
have been duly authorized, executed and delivered by the Company or such
Subsidiary, constitute valid and binding agreements of the Company or such
Subsidiary and are enforceable against the Company or such Subsidiary in
accordance with the terms thereof.

          (q) No statement, representation, warranty or covenant made by the
Company in this Agreement or the Indenture or made in any certificate or
document required by this Agreement to be delivered to you was or will be, when
made, inaccurate, untrue or incorrect in any material respect.

          (r) Neither the Company nor any of its directors, officers or
controlling persons has taken, directly or indirectly, any action designed, or
which might reasonably be expected, to cause or result, under the Act or
otherwise, in, or which has constituted, stabilization or manipulation of the
price of any security of the Company to facilitate the sale or resale of the
Securities.

          (s) No holder of securities of the Company has rights to the
registration of any securities of the Company because of the filing of the
Registration Statement.

          (t) The Company and its Subsidiaries are in substantial compliance
with all federal, state and local employment and labor laws, including, but not
limited to, laws relating to non-discrimination in hiring, promotion and pay of
employees; no labor dispute with the employees of the Company or any Subsidiary
exists or, to the knowledge of the Company, is imminent or threatened; and the
Company is not aware of any existing, imminent or threatened labor disturbance
by the employees of any of its principal suppliers, manufacturers or contractors
that could result in a Material Adverse Effect.

          (u) The Company and its Subsidiaries own, or are licensed or otherwise
have the full exclusive right to use, all material trademarks and trade names
which are used in or necessary for the conduct of their respective businesses as
described in the Prospectus. No claims have been asserted by any person to the
use of any such trademarks or trade names or challenging or questioning the
validity or effectiveness of any such trademark or trade name. The use, in
connection with the business and operations of the Company and its Subsidiaries
of such trademarks and trade names does not, to the Company's knowledge,
infringe on the rights of any person.

          (v) Neither the Company nor any of its Subsidiaries nor, to the
Company's knowledge, any employee or agent of the Company or any Subsidiary has
made any payment of funds of the Company or any Subsidiary or received or
retained any funds in violation of any law, rule or regulation or of a character
required to be disclosed in the Prospectus.

                                       7
<PAGE>
 
          (w) All United States federal income tax returns of the Company and
its Subsidiaries required by law to be filed have been filed, and all other
franchise and income tax returns of the Company and its Subsidiaries required to
be filed pursuant to applicable foreign, state or local law have been filed, and
all taxes shown by such returns or otherwise assessed, which are due and
payable, have been paid, except tax assessments, if any, as are being contested
in good faith and as to which adequate reserves have been provided. The charges,
accruals and reserves on the books of the Company and its Subsidiaries in
respect of any income and corporate franchise tax liability for any years not
finally determined are adequate to meet any assessments or reassessments for
additional income or corporate franchise tax for any years not finally
determined.

          (x) The Company and its subsidiaries are implementing a comprehensive,
detailed program to analyze and address the risk that the computer hardware and
software used by them may be unable to recognize and properly execute date
sensitive functions involving certain dates prior to and any dates after
December 31, 1999 (the "Year 2000 Problem"), and reasonably believes that such
risk will be remedied on a timely basis without material expense and will not
have a Material Adverse Effect; and the Company believes, after due inquiry,
that each supplier, vendor, customer or financial service organization used or
serviced by the Company and its Subsidiaries has remedied or will remedy on a
timely basis the Year 2000 Problem, except to the extent that a failure to
remedy by any such supplier, vendor, customer or financial service organization
would not have a Material Adverse Effect. The Company is in compliance with the
Commission's most recent staff legal bulletin related to Year 2000 compliance.

          (y) The Company and its Subsidiaries (i) are in compliance with any
and all applicable foreign, federal, state and local laws and regulations
relating to the protection of human health and safety, the environment or
imposing liability or standards of conduct concerning any Hazardous Material (as
hereinafter defined) ("Environmental Laws"), (ii) have received all permits,
licenses or other approvals required of them under applicable Environmental Laws
to conduct their respective businesses and (iii) are in compliance with all
terms and conditions of any such permit, license or approval, except where such
noncompliance with Environmental Laws, failure to receive required permits,
licenses or other approvals or failure to comply with the terms and conditions
of such permits, licenses or approvals would not, individually or in the
aggregate, result in a Material Adverse Effect. The term "Hazardous Material"
means (A) any "hazardous substance" as defined by the Comprehensive
Environmental Response, Compensation and Liability Act of 1980, as amended, (B)
any "hazardous waste" as defined by the Resource Conservation and Recovery Act,
as amended, (C) any petroleum or petroleum product, (D) any polychlorinated
biphenyl and (E) any pollutant or contaminant or hazardous, dangerous, or toxic
chemical, material, waste or substance regulated under or within the meaning of
any other Environmental Law.

          (z) In the ordinary course of its business, the Company conducts a
periodic review of the effect of Environmental Laws on the business, operations
and properties of the Company and its Subsidiaries, in the course of which it
identifies and evaluates associated costs and liabilities (including, without
limitation, any capital or operating expenditures required for clean-up, closure
of properties or compliance with Environmental Laws or any permit, license or
approval, any related constraints on operating activities and any potential
liabilities to third parties). Except as set forth in the Registration Statement
and the Prospectus, there are no costs

                                       8
<PAGE>
 
and liabilities associated with or arising in connection with Environmental Laws
as currently in effect (including, without limitation, costs of compliance
therewith) which would, singly or in the aggregate have a Material Adverse
Effect.

          (aa)  The Company maintains insurance with respect to its properties
and business of the types and in amounts generally deemed adequate for its
business and consistent with insurance coverage maintained by similar companies
and businesses, all of which insurance is in full force and effect.

          (bb)   With respect to each employee benefit plan, program and
arrangement (including, without limitation, any "employee benefit plan" as
defined in Section 3(3) of the Employee Retirement Income Security Act of 1974,
as amended ("ERISA")) maintained or contributed to by the Company, or with
respect to which the Company could incur any liability under ERISA
(collectively, the "Benefit Plans"), no event has occurred and, to the best
knowledge of the Company, there exists no condition or set of circumstances, in
connection with which the Company could be subject to any liability under the
terms of such Benefit Plan, applicable law (including, without limitation, ERISA
and the Internal Revenue Code of 1986, as amended) or any applicable agreement
that could have a Material Adverse Effect.

     4.   Agreements of the Company.  The Company agrees with you as follows:
          
          (a) The Company will not, during such period as the Prospectus is
required by law to be delivered in connection with sales of the Securities by
you or a dealer, file any amendment or supplement to the Registration Statement
or the Prospectus, unless a copy thereof shall first have been submitted to you
within a reasonable period of time prior to the filing thereof and you shall not
have objected thereto in good faith.

          (b) The Company will notify you promptly, and will confirm such advice
in writing, (i) when any post-effective amendment to the Registration Statement
becomes effective, (ii) of any request by the Commission for amendments or
supplements to the Registration Statement or the Prospectus or for additional
information, (iii) of the issuance by the Commission of any stop order
suspending the effectiveness of the Registration Statement or the initiation of
any proceedings for that purpose or the threat thereof, (iv) of the happening of
any event during the period mentioned in the second sentence of Section 4(e)
that in the judgment of the Company makes any statement made in the Registration
Statement or the Prospectus untrue or that requires the making of any changes in
the Registration Statement or the Prospectus in order to make the statements
therein, in light of the circumstances in which they are made, not misleading
and (v) of receipt by the Company or any representative or attorney of the
Company of any other communication from the Commission relating to the Company,
the Registration Statement, any preliminary prospectus or the Prospectus. If at
any time the Commission shall issue any order suspending the effectiveness of
the Registration Statement, the Company will make every reasonable effort to
obtain the withdrawal of such order at the earliest possible moment. The Company
will use its best efforts to comply with the provisions of and make all
requisite filings with the Commission pursuant to Rule 430A and to notify you
promptly of all such filings.

          (c) The Company has furnished, or will furnish, to you, without
charge, two conformed copies of the Registration Statement and of any post-
effective amendment thereto,

                                       9
<PAGE>
 
including financial statements and schedules, and all exhibits thereto
(including any document filed under the Exchange Act and deemed to be
incorporated by reference into the Prospectus).

          (d) The Company will comply with all the provisions of any
undertakings contained in the Registration Statement.

          (e) On the Effective Date, and thereafter from time to time, the
Company will deliver to the Underwriter, without charge, as many copies of the
Prospectus or any amendment or supplement thereto, as you may reasonably
request. The Company consents to the use of the Prospectus or any amendment or
supplement thereto by the Underwriter and by all dealers to whom the Securities
may be sold, both in connection with the offering or sale of the Securities and
for any period of time thereafter during which the Prospectus is required by law
to be delivered in connection therewith. If during such period of time any event
shall occur which in the judgment of the Company or counsel to the Underwriter
should be set forth in the Prospectus in order to make any statement therein, in
the light of the circumstances under which it was made, not misleading, or if it
is necessary to supplement or amend the Prospectus to comply with law, the
Company will forthwith prepare and duly file with the Commission an appropriate
supplement or amendment thereto, and will deliver to the Underwriter, without
charge, such number of copies thereof as you may reasonably request. The Company
shall not file any document under the Exchange Act before the termination of the
offering of the Securities by you if such document would be deemed to be
incorporated by reference into the Prospectus and if such document is not
approved by the Underwriter after reasonable notice thereof.

          (f) Prior to any public offering of the Securities by the Underwriter,
the Company will cooperate with the Underwriter and the Underwriter's counsel in
connection with the registration or qualification of the Securities for offer
and sale under the securities or Blue Sky laws of such jurisdictions as you may
request; provided, that in no event shall the Company be obligated to qualify to
do business in any jurisdiction where it is not now so qualified or to take any
action which would subject it to general service of process in any jurisdiction
where it is not now so subject.

          (g) During the period of five years commencing on the date of the
Price Determination Agreement, the Company will furnish to you copies of such
financial statements and other periodic and special reports as the Company may
from time to time distribute generally to the holders of any class of its
capital stock, and will furnish to you a copy of each annual or other report it
shall be required to file with the Commission.

          (h) The Company will make generally available to holders of its
securities as soon as may be practicable but in no event later than the last day
of the fifteenth full calendar month following the calendar quarter in which the
"effective date of the Registration Statement" (as defined in Rule 158 of the
Rules and Regulations) falls, an earnings statement (which need not be audited
but shall be in reasonable detail) for a period of 12 months ended commencing
after such "effective date of the Registration Statement," and satisfying the
provisions of Section 11(a) of the Act (including Rule 158 of the Rules and
Regulations).

          (i) Whether or not the transactions contemplated by this Agreement are
consummated or this Agreement is terminated, the Company will pay, or reimburse
if paid by you, all costs and expenses incident to the performance of the
obligations of the Company under

                                      10
<PAGE>
 
this Agreement, including but not limited to costs and expenses of or relating
to (i) the preparation, printing and filing of the Registration Statement and
exhibits to it, each preliminary prospectus, the Prospectus, any amendment or
supplement to the Registration Statement or the Prospectus and the Indenture,
(ii) the preparation and delivery of certificates representing the Securities,
(iii) the printing of this Agreement and any Dealer Agreements, (iv) furnishing
(including costs of shipping, mailing and courier) such copies of the
Registration Statement, the Prospectus and any preliminary prospectus, and all
amendments and supplements thereto, as may be requested for use in connection
with the offering and sale of the Securities by the Underwriters or by dealers
to whom Securities may be sold, (v) any filings required to be made by the
Underwriter with the NASD, and the fees, disbursements and other charges of
counsel for the Underwriter in connection therewith, (vi) the registration or
qualification of the Securities for offer and sale under the securities or Blue
Sky laws of such jurisdictions designated pursuant to Section 4(f), including
the fees, disbursements and other charges of counsel to the Underwriter in
connection therewith, and the preparation and printing of preliminary,
supplemental and final Blue Sky memoranda, (vii) counsel to the Company, (viii)
the transfer agent and registrar for the Securities, (ix) the rating of the
Securities by one or more rating agencies, (x) the Trustee and any agent of the
Trustee and the fees, disbursements and other charges of counsel for the Trustee
in connection with the Indenture and the Securities and (xi) the Accountants.
Except as otherwise provided in this Section 4, the Underwriter shall pay all of
its own costs and expenses.

          (j) If this Agreement shall be terminated by the Company pursuant to
any of the provisions hereof or if for any reason the Company shall be unable to
perform its obligations hereunder, the Company will reimburse the Underwriter
for all out- of-pocket expenses (including the fees, disbursements and other
charges of counsel to the Underwriter) reasonably incurred by it in connection
herewith.

          (k) The Company will not at any time, directly or indirectly, take any
action intended, or which might reasonably be expected, to cause or result in,
or which will constitute, stabilization of the price of the Securities to
facilitate the sale or resale of any of the Securities.

          (l) The Company will apply the net proceeds from the offering and sale
of the Securities in the manner set forth in the Prospectus under "Use of
Proceeds."

          (m) The Company will not claim the benefit of any usury law against
any holders of Securities.

     5.   Conditions of Obligations of the Underwriters. In addition to the
execution and delivery of the Price Determination Agreement, the obligations of
the Underwriter hereunder are subject to the following conditions:

          (a) (i) No stop order suspending the effectiveness of the Registration
Statement shall have been issued and no proceedings for that purpose shall be
pending or threatened by the Commission, (ii) no order suspending the
effectiveness of the Registration Statement or the qualification or registration
of the Securities under the securities or Blue Sky laws of any jurisdiction
shall be in effect and no proceeding for such purpose shall be pending before or
threatened or contemplated by the Commission or the authorities of any such
jurisdiction, (iii) any request for additional information on the part of the
staff of the Commission or any such authorities shall have been complied with to
the satisfaction of the staff of the

                                      11
<PAGE>
 
Commission or such authorities and (iv) after the date hereof no amendment or
supplement to the Registration Statement or the Prospectus shall have been filed
unless a copy thereof was first submitted to you and you did not object thereto
in good faith, and you shall have received certificates, dated the Closing Date
and signed by the Chief Executive Officer, President or a Vice President of the
Company and the Chief Financial Officer of the Company (who may, as to
proceedings threatened, rely upon the best of their information and belief), to
the effect of clauses (i), (ii) and (iii).

          (b) Since the respective dates as of which information is given in the
Registration Statement and the Prospectus (i) there shall not have been a
material adverse change in the general affairs, business, business prospects,
properties, management, condition (financial or otherwise) or results of
operations of the Company and its Subsidiaries, taken as a whole, whether or not
arising from transactions in the ordinary course of business, in each case other
than as set forth in or contemplated by the Registration Statement and the
Prospectus, and (ii) neither the Company nor any of its Subsidiaries shall have
sustained any material loss or interference with its business or properties from
fire, explosion, flood or other casualty, whether or not covered by insurance,
or from any labor dispute or any court or legislative or other governmental
action, order or decree, which is not set forth in the Registration Statement
and the Prospectus, if in your judgment any such development makes it
impracticable or inadvisable to consummate the sale and delivery of the
Securities by the Underwriter in accordance with the terms hereof and thereof.

          (c) Since the respective dates as of which information is given in the
Registration Statement and the Prospectus, there shall have been no litigation
or other proceeding instituted against the Company or any of its Subsidiaries or
any of their respective officers or directors in their capacities as such,
before or by any Federal, state or local court, commission, regulatory body,
administrative agency or other governmental body, domestic or foreign, in which
litigation or proceeding an unfavorable ruling, decision or finding would
materially and adversely affect the business, properties, business prospects,
condition (financial or otherwise) or results of operations of the Company and
its Subsidiaries taken as a whole.

          (d) Each of the representations and warranties of the Company
contained herein shall be true and correct in all material respects at the
Closing Date, as if made and all covenants and agreements herein contained to be
performed on the part of the Company and all conditions herein contained to be
fulfilled or complied with by the Company at or prior to the Closing Date shall
have been duly performed, fulfilled or complied with.

          (e) The Underwriter shall have received opinions, each dated the
Closing Date and satisfactory in form and substance to counsel for you, from
Gable & Gotwals, counsel to the Company, to the effect set forth in Exhibit B
and Andersen, Byrd, Richardson, Flarety and Hendricks, special counsel to the
Company.

          (f) The Underwriter shall have received an opinion, dated the Closing
Date from Jones, Day, Reavis & Pogue, counsel to the Underwriter, with respect
to the Registration Statement, the Prospectus and this Agreement, which opinion
shall be satisfactory in all respects to you.

                                       12
<PAGE>
 
          (g) On the date of the Price Determination Agreement, the Accountants
shall have furnished to the Underwriter letters, each dated the date of its
delivery, addressed to the Underwriter and in form and substance satisfactory to
the Underwriter, confirming that they are independent accountants with respect
to the Company or Southwest Gas, as the case may be, as required by the Act and
the Rules and Regulations and with respect to the financial and other
statistical and numerical information contained or incorporated by reference in
the Registration Statement.  At the Closing Date, the Accountants shall have
furnished to the Underwriter letters, each dated the date of its delivery, which
shall confirm, on the basis of a review in accordance with the procedures set
forth in the respective letters from the Accountants, that nothing has come to
their attention during the period from the date of the letter referred to in the
prior sentence to a date (specified in the letter) not more than five days prior
to the Closing Date which would require any change in their letter dated the
date of the Price Determination Agreement, if it were required to be dated and
delivered at the Closing Date.

          (h) At the Closing Date, there shall be furnished to the Underwriter
an accurate certificate, dated the date of its delivery, signed by each of the
Chief Executive Officer, President or a Vice President and the Chief Financial
Officer of the Company, in form and substance satisfactory to you, to the effect
that:

               (i) Each signer of such certificate has carefully examined the
Registration Statement and the Prospectus (including any documents filed under
the Exchange Act and deemed to be incorporated by reference into the Prospectus)
and (A) as of the date of such certificate, such documents are true and correct
in all material respects and do not omit to state a material fact required to be
stated therein or necessary in order to make the statements therein not untrue
or misleading and (B) since the Effective Date, no event has occurred as a
result of which it is necessary to amend or supplement the Prospectus in order
to make the statements therein not untrue or misleading in any material respect
and there has been no document required to be filed under the Exchange Act and
the Exchange Act Rules and Regulations that upon such filing would be deemed to
be incorporated by reference into the Prospectus that has not been so filed.

               (ii) Each of the representations and warranties of the Company
contained in this Agreement were, when originally made, and are, at the time
such certificate is delivered, true and correct in all material respects.

               (iii) Each of the covenants required herein to be performed by
the Company on or prior to the delivery of such certificate has been duly,
timely and fully performed and each condition herein required to be complied
with by the Company on or prior to the date of such certificate has been duly,
timely and fully complied with.

          (i) The Securities shall be qualified for sale in such states as the
Underwriter may reasonably request, each such qualification shall be in effect
and not subject to any stop order or other proceeding on the Closing Date.

          (j) Subsequent to the execution and delivery of this Agreement and
prior to the Closing Date, there shall not have been any downgrading, nor any
notice given of any intended or potential downgrading or of a possible change
that does not indicate the direction of the possible change, in the rating
accorded any of the Company's securities, including the

                                       13
<PAGE>
 
Securities, by any "nationally recognized statistical rating organization," as
such term is defined for purposes of Rule 436(g)(2) under the 1933 Act.

          (k) The Company shall have furnished to the Underwriter such
certificates, in addition to those specifically mentioned herein, as the
Underwriter may have reasonably requested as to the accuracy and completeness at
the Closing Date of any statement in the Registration Statement or the
Prospectus or any documents filed under the Exchange Act and deemed to be
incorporated by reference into the Prospectus, as to the accuracy at the Closing
Date of the representations and warranties of the Company herein, as to the
performance by the Company of its obligations hereunder, or as to the
fulfillment of the conditions concurrent and precedent to the obligations
hereunder of the Underwriter.

     6.   Indemnification.

          (a) The Company will indemnify and hold harmless the Underwriter, the
directors, officers, employees and agents of the Underwriter and each person, if
any, who controls the Underwriter within the meaning of Section 15 of the Act or
Section 20 of the Exchange Act from and against any and all losses, claims,
liabilities, expenses and damages (including, but not limited to, any and all
investigative, legal and other expenses reasonably incurred in connection with,
and any and all amounts paid in settlement of, any action, suit or proceeding
between any of the indemnified parties and any indemnifying parties or between
any indemnified party and any third party, or otherwise, or any claim asserted),
as and when incurred, to which the Underwriter, or any such person, may become
subject under the Act, the Exchange Act or other Federal or state statutory law
or regulation, at common law or otherwise, insofar as such losses, claims,
liabilities, expenses or damages arise out of or are based on (i) any untrue
statement or alleged untrue statement of a material fact contained in any
preliminary prospectus, the Registration Statement or the Prospectus or any
amendment or supplement to the Registration Statement or the Prospectus or in
any documents filed under the Exchange Act and deemed to be incorporated by
reference into the Prospectus by or on behalf of the Company or based on written
information furnished by or on behalf of the Company filed in any jurisdiction
in order to qualify the Securities under the securities laws thereof or filed
with the Commission, (ii) the omission or alleged omission to state in such
document a material fact required to be stated in it or necessary to make the
statements in it not misleading or (iii) any act or failure to act or any
alleged act or failure to act by the Underwriter in connection with, or relating
in any manner to, the Securities or the offering contemplated hereby, and which
is included as part of or referred to in any loss, claim, liability, expense or
damage arising out of or based upon matters covered by clause (i) or (ii) above
(provided that the Company shall not be liable under this clause (iii) to the
extent it is finally judicially determined by a court of competent jurisdiction
that such loss, claim, liability, expense or damage resulted directly from any
such acts or failures to act undertaken or omitted to be taken by such
underwriter through its gross negligence or willful misconduct); provided that
the Company will not be liable to the extent that such loss, claim, liability,
expense or damage arises from the sale of the Securities in the public offering
to any person by the Underwriter and is based on an untrue statement or omission
or alleged untrue statement or omission made in reliance on and in conformity
with information relating to the Underwriter furnished in writing to the Company
by the Underwriter expressly for inclusion in the Registration Statement, any
preliminary prospectus or the Prospectus.  This indemnity agreement will be in
addition to any liability that the Company might otherwise have.

                                       14
<PAGE>
 
          (b) The Underwriter will indemnify and hold harmless the Company, each
person, if any, who controls the Company within the meaning of Section 15 of the
Act or Section 20 of the Exchange Act, each director of the Company and each
officer of the Company who signs the Registration Statement to the same extent
as the foregoing indemnity from the Company to the Underwriter, but only insofar
as losses, claims, liabilities, expenses or damages arise out of or are based on
any untrue statement or omission or alleged untrue statement or omission made in
reliance on and in conformity with information relating to the Underwriter
furnished in writing to the Company by the Underwriter expressly for use in the
Registration Statement, any preliminary prospectus or the Prospectus.  This
indemnity agreement will be in addition to any liability that the Underwriter
might otherwise have; provided, however, that in no case shall the Underwriter
be liable or responsible for any amount in excess of the underwriting discounts
and commissions received by the Underwriter.

          (c) Any party that proposes to assert the right to be indemnified
under this Section 6 will, promptly after receipt of notice of commencement of
any action against such party in respect of which a claim is to be made against
an indemnifying party or parties under this Section 6, notify each such
indemnifying party of the commencement of such action, enclosing a copy of all
papers served, but the omission so to notify such indemnifying party will not
relieve it from any liability that it may have to any indemnified party under
the foregoing provisions of this Section 6 unless, and only to the extent that,
such omission results in the forfeiture of substantive rights or defenses by the
indemnifying party.  If any such action is brought against any indemnified party
and it notifies the indemnifying party of its commencement, the indemnifying
party will be entitled to participate in and, to the extent that it elects by
delivering written notice to the indemnified party promptly after receiving
notice of the commencement of the action from the indemnified party to assume
the defense of the action, with counsel satisfactory to the indemnified party,
and after notice from the indemnifying party to the indemnified party of its
election to assume the defense, the indemnifying party will not be liable to the
indemnified party for any legal or other expenses except as provided below and
except for the reasonable costs of investigation subsequently incurred by the
indemnified party in connection with the defense.  The indemnified party will
have the right to employ its own counsel in any such action, but the fees,
expenses and other charges of such counsel will be at the expense of such
indemnified party unless (i) the employment of counsel by the indemnified party
has been authorized in writing by the indemnifying party, (ii) the indemnified
party has reasonably concluded (based on advice of counsel) that there may be
legal defenses available to it or other indemnified parties that are different
from or in addition to those available to the indemnifying party, (iii) a
conflict or potential conflict exists (based on advice of counsel to the
indemnified party) between the indemnified party and the indemnifying party (in
which case the indemnifying party will not have the right to direct the defense
of such action on behalf of the indemnified party) or (iv) the indemnifying
party has not in fact employed counsel to assume the defense of such action
within a reasonable time after receiving notice of the commencement of the
action, in each of which cases the reasonable fees, disbursements and other
charges of counsel will be at the expense of the indemnifying party or parties.
It is understood that the indemnifying party shall not, in connection with any
proceeding or related proceedings in the same jurisdiction, be liable for the
reasonable fees, disbursements and other charges of more than one separate firm
admitted to practice in such jurisdiction at any one time for all such
indemnified party or parties.  All such fees, disbursements and other charges
will be reimbursed by the indemnifying party promptly as they are incurred.  An
indemnifying party will not be liable for any settlement of any action or claim
effected without its written consent (which

                                       15
<PAGE>
 
consent will not be unreasonably withheld). No indemnifying party shall, without
the prior written consent of each indemnified party, settle or compromise or
consent to the entry of any judgment in any pending or threatened claim, action
or proceeding relating to the matters contemplated by this Section 6 (whether or
not any indemnified party is a party thereto), unless such settlement,
compromise or consent includes an unconditional release of each indemnified
party from all liability arising or that may arise out of such claim, action or
proceeding. Notwithstanding any other provision of this Section 6(c), if at any
time an indemnified party shall have requested an indemnifying party to
reimburse the indemnified party for fees and expenses of counsel, such
indemnifying party agrees that it shall be liable for any settlement effected
without its written consent if (1) such settlement is entered into more than 45
days after receipt by such indemnifying party of the aforesaid request, (2) such
indemnifying party shall have received notice of the terms of such settlement at
least 30 days prior to such settlement being entered into and (3) such
indemnifying party shall not have reimbursed such indemnified party in
accordance with such request prior to the date of such settlement.

          (d) In order to provide for just and equitable contribution in
circumstances in which the indemnification provided for in the foregoing
paragraphs of this Section 6 is applicable in accordance with its terms but for
any reason is held to be unavailable from the Company or the Underwriter, the
Company and the Underwriter will contribute to the total losses, claims,
liabilities, expenses and damages (including any investigative, legal and other
expenses reasonably incurred in connection with, and any amount paid in
settlement of, any action, suit or proceeding or any claim asserted, but after
deducting any contribution received by the Company from persons other than the
Underwriter, such as persons who control the Company within the meaning of the
Act, officers of the Company who signed the Registration Statement and directors
of the Company, who also may be liable for contribution) to which the Company
and the Underwriter may be subject in such proportion as shall be appropriate to
reflect the relative benefits received by the Company on the one hand and the
Underwriter on the other.  The relative benefits received by the Company on the
one hand and the Underwriter on the other shall be deemed to be in the same
proportion as the total net proceeds from the offering (before deducting
expenses) received by the Company bear to the total underwriting discounts and
commissions received by the Underwriter, in each case as described in the
Prospectus.  If, but only if, the allocation provided by the foregoing sentence
is not permitted by applicable law, the allocation of contribution shall be made
in such proportion as is appropriate to reflect not only the relative benefits
referred to in the foregoing sentence but also the relative fault of the
Company, on the one hand, and the Underwriter, on the other, with respect to the
statements or omissions which resulted in such loss, claim, liability, expense
or damage, or action in respect thereof, as well as any other relevant equitable
considerations with respect to such offering.  Such relative fault shall be
determined by reference to whether the untrue or alleged untrue statement of a
material fact or omission or alleged omission to state a material fact relates
to information supplied by the Company or the Underwriter, the intent of the
parties and their relative knowledge, access to information and opportunity to
correct or prevent such statement or omission.  The Company and the Underwriter
agree that it would not be just and equitable if contributions pursuant to this
Section 6(d) were to be determined by pro rata allocation or by any other method
of allocation which does not take into account the equitable considerations
referred to herein.  The amount paid or payable by an indemnified party as a
result of the loss, claim, liability, expense or damage, or action in respect
thereof, referred to above in this Section 6(d) shall be deemed to include, for
purpose of this Section 6(d), any legal or other expenses reasonably incurred by
such indemnified party in connection with investigating or defending any

                                       16
<PAGE>
 
such action or claim.  Notwithstanding the provisions of this Section 6(d), the
Underwriter shall not be required to contribute any amount in excess of the
underwriting discounts and commissions received by it and no person found guilty
of fraudulent misrepresentation (within the meaning of Section 11(f) of the Act)
will be entitled to contribution from any person who was not guilty of such
fraudulent misrepresentation.  For purposes of this Section 6(d), any person who
controls a party to this Agreement within the meaning of the Act will have the
same rights to contribution as that party, and each officer of the Company who
signed the Registration Statement will have the same rights to contribution as
the Company, subject in each case to the provisions hereof.  Any party entitled
to contribution, promptly after receipt of notice of commencement of any action
against such party in respect of which a claim for contribution may be made
under this Section 6(d), will notify any such party or parties from whom
contribution may be sought, but the omission so to notify will not relieve the
party or parties from whom contribution may be sought from any other obligation
it or they may have under this Section 6(d). Except for a settlement entered
into pursuant to the last sentence of Section 6(c) hereof, no party will be
liable for contribution with respect to any action or claim settled without its
written consent (which consent will not be unreasonably withheld).

          (e) The indemnity and contribution agreements contained in this
Section 6 and the representations and warranties of the Company contained in
this Agreement shall remain operative and in full force and effect regardless of
(i) any investigation made by or on behalf of the Underwriter, (ii) acceptance
of any of the Securities and payment therefor or (iii) any termination of this
Agreement.

     7.   Termination.  The obligations of the Underwriter under this Agreement
may be terminated at any time on or prior to the Closing Date by notice to the
Company from you, without liability on the part of the Underwriter to the
Company, if, prior to delivery and payment for the Securities in the sole
judgment of the Underwriter, (a) trading in securities generally on the New York
Stock Exchange shall have been suspended or limited or minimum or maximum prices
shall have been generally established on such exchange, or additional material
governmental restrictions, not in force on the date of this Agreement, shall
have been imposed upon trading in securities generally by such exchange or by
order of the Commission or any court or other governmental authority, (b) a
general banking moratorium shall have been declared by either Federal or New
York State authorities or (c) any material adverse change in the financial or
securities markets in the United States or in political, financial or economic
conditions in the United States or any outbreak or material escalation of
hostilities or declaration by the United States of a national emergency or war
or other calamity or crisis shall have occurred the effect of any of which is
such as to make it, in the sole judgment of the Underwriter, impracticable or
inadvisable to market the Securities on the terms and in the manner contemplated
by the Prospectus.

     8.   Miscellaneous.  Notice given pursuant to any of the provisions of this
Agreement shall be in writing and, unless otherwise specified, shall be mailed
or delivered (a) if to the Company, at the office of the Company, 100 West Fifth
Street, Tulsa, Oklahoma 74103, Attention: Chief Financial Officer or (b) if to
the Underwriter at the offices of PaineWebber Incorporated, 1285 Avenue of the
Americas, New York, New York 10019, Attention: Corporate Finance Department.
Any such notice shall be effective only upon receipt.  Any notice under Section
7 may be made by telex or telephone, but if so made shall be subsequently
confirmed in writing.

                                       17
<PAGE>
 
     This Agreement has been and is made solely for the benefit of the
Underwriter and the Company and of the controlling persons, directors and
officers referred to in Section 6, and their respective successors and assigns,
and no other person shall acquire or have any right under or by virtue of this
Agreement.  The term "successors and assigns" as used in this Agreement shall
not include a purchaser, as such purchaser, of Securities from the Underwriter.

     All representations, warranties and agreements of the Company contained
herein or in certificates or other instruments delivered pursuant hereto, shall
remain operative and in full force and effect regardless of any investigation
made by or on behalf of the Underwriter or any of its controlling persons and
shall survive delivery of and payment for the Securities hereunder.

     THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE
LAWS OF THE STATE OF NEW YORK WITHOUT REGARD TO THE CONFLICT OF LAWS PRINCIPLES
OF SUCH STATE.

     This Agreement may be signed in two or more counterparts with the same
effect as if the signatures thereto and hereto were upon the same instrument.
In case any provision in this Agreement shall be invalid, illegal or
unenforceable, the validity, legality and enforceability of the remaining
provisions shall not in any way be affected or impaired thereby.

     The Company and the Underwriter each hereby irrevocably waive any right
they may have to trial by jury in respect of any claim based upon or arising out
of this Agreement or the transactions contemplated hereby.

     This Agreement may not be amended or otherwise modified or any provision
hereof waived except by an instrument in writing signed by the Underwriter and
the Company.


                            [signature page follows]

                                       18
<PAGE>
 
     Please confirm that the foregoing correctly sets forth the agreement
between the Company and the Underwriter.

                                           Very truly yours,

                                           ONEOK, INC.


                                           By: /s/ Jerry D. Neal
                                               _______________________________
                                           Title: Vice President and
                                                  Chief Financial Officer


Confirmed as of the date first
above mentioned:


PAINEWEBBER INCORPORATED


By: /s/ Peter Masco
    ___________________________
Title: Managing Director

                                       19
<PAGE>
 
                                                                       EXHIBIT A



                                  ONEOK, INC.



                         PRICE DETERMINATION AGREEMENT
                         -----------------------------


                                                                February 3, 1999



PAINEWEBBER INCORPORATED
1285 Avenue of the Americas
New York, New York 10019

Dear Sirs:

     Reference is made to the Underwriting Agreement, dated February 3, 1999
(the "Underwriting Agreement"), between ONEOK, Inc., an Oklahoma corporation
(the "Company"), and PaineWebber Incorporated ("you" or the "Underwriter").  The
Underwriting Agreement provides for the purchase by the Underwriter from the
Company, subject to the terms and conditions set forth therein, of an aggregate
of $100,000,000 principal amount of the Company's 6% Debentures due February 1,
2009 (the "Securities") to be issued pursuant to an Indenture dated as of
September 24, 1998 between the Company and Chase Bank of Texas, National
Association, as Trustee.  This Agreement is the Price Determination Agreement
referred to in the Underwriting Agreement.

     Pursuant to Section 1 of the Underwriting Agreement, the undersigned agree
with you that the purchase price for the Securities to be paid by you shall be
98.894% of the aggregate principal amount of the Securities.

     The Company represents and warrants to you that the representations and
warranties of the Company set forth in Section 3 of the Underwriting Agreement
are accurate as though expressly made at and as of the date hereof.

     This Agreement shall be governed by the law of the State of New York
without regard to the conflict of law principles of such State.

     If the foregoing is in accordance with your understanding of the agreement
between the Underwriter and the Company, please sign and return to the Company a
counterpart hereof, whereupon this instrument along with all counterparts and
together with the Underwriting

                                      A-1
<PAGE>
 
Agreement shall be a binding agreement between the Underwriter and the Company
in accordance with its terms and the terms of the Underwriting Agreement.

                              Very truly yours,



                              ONEOK, INC.


                              By: _____________________________________________
                              Title: Vice President and Chief Financial Officer



Confirmed as of the date
 first above mentioned:


PAINEWEBBER INCORPORATED


By: ________________________
Title: Managing Director

                                      A-2
<PAGE>
 
                                                                       EXHIBIT B



                               Form of Opinion of
                             Counsel to the Company
                             ----------------------


     1.   The Company and each of its Subsidiaries is a corporation or limited
liability company duly organized, validly existing and in good standing under
the laws of the jurisdiction of its incorporation or organization and has full
corporate power and authority to conduct all the activities conducted by it, to
own or lease all the assets owned or leased by it and to conduct its business as
described in the Registration Statement and the Prospectus.  The Company is the
sole record owner and, to our knowledge, the sole beneficial owner of all of the
capital stock or capital interests of each of its Subsidiaries.

     2.   No consent, approval, authorization or order of, or any filing or
declaration with, any court or governmental agency or body is required in
connection with the authorization, issuance, transfer, sale or delivery of the
Securities, in connection with the execution, delivery and performance of the
Agreement by the Company or in connection with the taking by the Company of any
action contemplated thereby except such as have been obtained under the Act, the
Trust Indenture Act and the Rules and Regulations and such as may be required
under state securities or "Blue Sky" laws or by the by-laws and rules of the
NASD in connection with the purchase and distribution by the Underwriters of the
Securities.

     3.   The Registration Statement and the Prospectus (including any documents
incorporated by reference into the Prospectus, at the time they were filed)
comply or complied in all material respects as to form with the requirements of
the Act, the Rules and Regulations, the Exchange Act, the Exchange Act Rules and
Regulations, the Trust Indenture Act and the Trust Indenture Act Rules and
Regulations and the Indenture complies in all material respects as to form with
the Trust Indenture Act (except that such counsel need not express an opinion as
to (a) financial statements, schedules and other financial data contained in the
Registration Statement or the Prospectus (or incorporated by reference therein)
and (b) the Statement of Eligibility and Qualification under the Trust Indenture
Act of the Trustee on Form T-1).

     4.   To the best of such counsel's knowledge, any instrument, document,
lease, license, contract or other agreement (collectively, "Documents") required
to be described or referred to in the Registration Statement or the Prospectus
has been properly described or referred to therein and any Document required to
be filed as an exhibit to the Registration Statement has been filed as an
exhibit thereto or has been incorporated as an exhibit by reference in the
Registration Statement; and no default exists in the due performance or
observance of any material obligation, agreement, covenant or condition
contained in any Document filed or required to be filed as an exhibit to the
Registration Statement.

     5.   To the best of such counsel's knowledge, no person or entity has the
right to require the registration under the Act of shares of common stock or
other securities of the Company by reason of the filing or effectiveness of the
Registration Statement.

                                      B-1
<PAGE>
 
     6.   To the best of such counsel's knowledge, the Company is not in
violation of, or in default with respect to, any law, rule, regulation, order,
judgment or decree, except as may be described in the Prospectus or such as in
the aggregate do not now have and will not in the future have a Material Adverse
Effect.

     7.   The Securities and the Indenture conform in all material respects as
to legal matters to the description thereof contained in the Registration
Statement and the Prospectus.  All descriptions in the Prospectus of statutes,
regulations or legal or governmental proceedings are accurate and fairly present
the information required to be shown in all material respects.

     8.   The Company has full corporate power and authority to enter into the
Agreement, and the Agreement has been duly authorized, executed and delivered by
the Company, is a valid and binding agreement of the Company and, except for the
indemnification and contribution provisions thereof, as to which such counsel
need not express an opinion, is enforceable against the Company in accordance
with the terms thereof, subject to bankruptcy, insolvency, reorganization,
fraudulent transfer or conveyance or other laws of general applicability
relating to or affecting creditors' rights and to general principles of equity,
whether such principles are considered in a proceeding at law or equity.

     9.   The Company has full corporate power and authority to enter into the
Indenture and to issue the Securities, and the Indenture has been duly
authorized, executed and delivered by the Company and duly qualified under the
Trust Indenture Act; the Securities have been duly authorized, executed,
authenticated (assuming due authentication by or on behalf of the Trustee),
issued and delivered and are entitled to the benefits of the Indenture; and,
subject to bankruptcy, insolvency, reorganization, fraudulent transfer or
conveyance or other laws of general applicability relating to or affecting
creditors' rights and to general principles of equity, whether such principles
are considered in a proceeding at law or equity, each of the Indenture and the
Securities are legal, valid and binding obligations of the Company and are
enforceable against the Company in accordance with their respective terms.

     10.  The execution and delivery by the Company of, and the performance by
the Company of its agreements in, the Agreement, the Indenture and the
Securities do not and will not (a) violate the certificate of incorporation or
by-laws of the Company, (b) breach or result in a default under, cause the time
for performance of any obligation to be accelerated under, or result in the
creation or imposition of any lien, charge or encumbrance upon any of the assets
of the Company or any of its Subsidiaries pursuant to the terms of, (i) any
indenture, mortgage, deed of trust, loan agreement, bond, debenture, note
agreement, capital lease or other evidence of indebtedness of which such counsel
has knowledge, (ii) any voting trust arrangement or any contract or other
agreement to which the Company is a party that restricts the ability of the
Company to issue securities or (iii) any Document filed as an exhibit to, or
incorporated as an exhibit by reference in, the Registration Statement, (c)
breach or otherwise violate any existing obligation of the Company under any
court or administrative order, judgment or decree of which such counsel has
knowledge or (d) violate applicable provisions of any statute or regulation in
the State of Oklahoma or of the United States.

     11. The Company is not an "investment company" or an "affiliated person"
of, or "promoter" or "principal underwriter" for, an "investment company," as
such terms are defined in the Investment Company Act of 1940, as amended.

                                      B-2
<PAGE>
 
     Such counsel hereby confirms to the Underwriter that such counsel has been
advised by the Commission that the Registration Statement has become effective
under the Act and that no order suspending the effectiveness of the Registration
Statement has been issued and no proceeding for that purpose has been instituted
or is threatened, pending or contemplated.

     Such counsel hereby further confirms to the Underwriter that there are no
actions, suits, proceedings or investigations pending or, to such counsel's
knowledge, overtly threatened in writing against the Company or any of its
Subsidiaries, or any of their respective officers or directors in their
capacities as such, before or by any court, governmental agency or arbitrator
which (i) seek to challenge the legality or enforceability of the Agreement, the
Indenture or the Securities, (ii) seek to challenge the legality or
enforceability of any of the Documents filed, or required to be filed, as
exhibits to the Registration Statement, (iii) seek damages or other remedies
with respect to any of the Documents filed, or required to be filed, as exhibits
to the Registration Statement, (iv) except as set forth in or contemplated by
the Registration Statement and the Prospectus, seek money damages in excess of
$1,000,000 or seek to impose criminal penalties upon the Company, any of its
Subsidiaries or any of their respective officers or directors in their
capacities as such and of which such counsel has knowledge or (v) seek to enjoin
any of the business activities of the Company or any of its Subsidiaries or the
transactions described in the Prospectus and of which such counsel has
knowledge.

     Such counsel has participated in the preparation of the Registration
Statement and the Prospectus and, without assuming any responsibility for the
accuracy, completeness or fairness of the statements contained in the
Registration Statement or the Prospectus or in any amendment or supplement
thereto or in any document incorporated by reference into the Prospectus,
nothing has come to such counsel's attention that causes such counsel to believe
that, both as of the Effective Date and as of the Closing Date, the Registration
Statement, or any amendment thereto, contained or contains any untrue statement
of a material fact or omitted or omits to state a material fact required to be
stated therein or necessary to make the statements therein not misleading or
that any Prospectus or any amendment or supplement thereto including any
documents incorporated by reference into the Prospectus, at the time such
Prospectus was issued, at the time any such amended or supplemented Prospectus
was issued, at the Closing Date, contained or contains any untrue statement of a
material fact or omitted or omits to state a material fact necessary in order to
make the statements therein, in the light of the circumstances in which they
were made, not misleading (except that such counsel need not express an opinion
as to (a) financial statements, schedules and other financial data contained in
the Registration Statement or the Prospectus (or incorporated by reference
therein) and (b) the Statement of Eligibility and Qualification under the Trust
Indenture Act of the Trustee on Form T-1).

     In rendering the foregoing opinion, counsel may rely, to the extent they
deem such reliance proper, on the opinions of their counsel as to matters
governed by the laws of jurisdictions other than the United States and the State
of Oklahoma, and as to matters of fact, upon certificates of officers of the
Company and of government officials; provided that such counsel shall state that
in such counsel's opinion that such counsel and the Underwriter are justified in
relying on such opinions of other counsel.  Copies of all such opinions and
certificates shall be furnished to counsel to the Underwriter on the Closing
Date.  For purposes of paragraph 2 and 10(d) above, such counsel has reviewed
only those statutes, rules and regulations, including regulatory rules and
regulations, that in such counsel's experience are

                                      B-3
<PAGE>
 
applicable to transactions of the type contemplated by the Agreement or the
Indenture or for the offering, issuance, sale or delivery of the Securities.

     Capitalized terms used in such opinion but not defined shall have the
meanings assigned to them in the Agreement.

                                      B-4

<PAGE>
 
                                                                  Exhibit (1)(b)



                                  ONEOK, INC.



                         PRICE DETERMINATION AGREEMENT
                         -----------------------------


                                                                February 3, 1999



PAINEWEBBER INCORPORATED
1285 Avenue of the Americas
New York, New York 10019

Dear Sirs:

     Reference is made to the Underwriting Agreement, dated February 3, 1999
(the "Underwriting Agreement"), between ONEOK, Inc., an Oklahoma corporation
(the "Company"), and PaineWebber Incorporated ("you" or the "Underwriter").  The
Underwriting Agreement provides for the purchase by the Underwriter from the
Company, subject to the terms and conditions set forth therein, of an aggregate
of $100,000,000 principal amount of the Company's 6% Debentures due February 1,
2009 (the "Securities") to be issued pursuant to an Indenture dated as of
September 24, 1998 between the Company and Chase Bank of Texas, National
Association, as Trustee.  This Agreement is the Price Determination Agreement
referred to in the Underwriting Agreement.

     Pursuant to Section 1 of the Underwriting Agreement, the undersigned agree
with you that the purchase price for the Securities to be paid by you shall be
98.894% of the aggregate principal amount of the Securities.

     The Company represents and warrants to you that the representations and
warranties of the Company set forth in Section 3 of the Underwriting Agreement
are accurate as though expressly made at and as of the date hereof.

     This Agreement shall be governed by the law of the State of New York
without regard to the conflict of law principles of such State.
<PAGE>
 
     If the foregoing is in accordance with your understanding of the agreement
between the Underwriter and the Company, please sign and return to the Company a
counterpart hereof, whereupon this instrument along with all counterparts and
together with the Underwriting Agreement shall be a binding agreement between
the Underwriter and the Company in accordance with its terms and the terms of
the Underwriting Agreement.

                              Very truly yours,



                              ONEOK, INC.


                              By: /s/ Jerry D. Neal
                                  _____________________________________________
                              Title: Vice President and Chief Financial Officer


Confirmed as of the date
 first above mentioned:


PAINEWEBBER INCORPORATED


By: /s/ Peter Masco
    __________________________
Title: Managing Director

                                       2

<PAGE>
 
                                                                     Exhibit (4)





================================================================================



                                  ONEOK, INC.

                                 6% DEBENTURES

                              Due February 1, 2009

                               ------------------

                          THIRD SUPPLEMENTAL INDENTURE

                          Dated as of February 8, 1999

                               ------------------

                   Chase Bank of Texas, National Association
                                    TRUSTEE



================================================================================
<PAGE>
 
     THIS THIRD SUPPLEMENTAL INDENTURE is made as of the 8th day of February,
1999, by and between ONEOK, INC., an Oklahoma corporation (the "Company"), and
CHASE BANK OF TEXAS, NATIONAL ASSOCIATION, a national banking association (the
"Trustee").

                              W I T N E S S E T H:

     WHEREAS, the Company has heretofore entered into an Indenture, dated as of
September 24, 1998 (the "Original Indenture"), with the Trustee;

     WHEREAS, the Original Indenture is incorporated herein by this reference
and the Original Indenture, as supplemented by this Third Supplemental
Indenture, is herein called the "Indenture";

     WHEREAS, under the Original Indenture, a new series of Securities may at
any time be established pursuant to a supplemental indenture executed by the
Company and the Trustee;

     WHEREAS, the Company proposes to create under the Indenture a new series of
Securities; and

     WHEREAS, all conditions necessary to authorize the execution and delivery
of this Third Supplemental Indenture and to make it a valid and binding
obligation of the Company have been done or performed.

     NOW, THEREFORE, in consideration of the agreements and obligations set
forth herein and for other good and valuable consideration, the sufficiency of
which is hereby acknowledged, the parties hereto hereby agree as follows:

                                   ARTICLE 1

                       6% DEBENTURES DUE FEBRUARY 1, 2009

     SECTION 101. Establishment. There is hereby established a new series of
Securities to be issued under the Indenture, to be designated as the Company's
6% Debentures Due February 1, 2009 (the "Debentures").

     There are to be authenticated and delivered $100,000,000 principal amount
of Debentures to be issued at 99.494% of principal amount. The Company shall
have the right to issue additional Debentures at any time upon compliance with
the provisions of the Indenture for additional amounts of Securities. The
Debentures shall be issued in definitive fully registered form.

     The Debentures shall be issued in the form of one Global Security in
substantially the form set out in Exhibit A hereto. The initial Depositary with
respect to the Debentures shall be the Depository Trust Company.

     The Company will not pay Additional Amounts, as defined in Section 1008 of
the Original Indenture.
<PAGE>
 
     The form of the Trustee's Certificate of Authentication for the Debentures
shall be in substantially the form set forth in Exhibit B hereto.

     Each Debenture shall be dated the date of authentication thereof and shall
bear interest from the Original Issue Date.

     The interest rate on the Debenture will not be reset pursuant to Section
308(b) of the Original Indenture and the Stated Maturity shall not be extended
pursuant to Section 309 of the Original Indenture.

     SECTION 102. Definitions. The following defined terms used herein shall,
unless the context otherwise requires, have the meanings specified below.
Capitalized terms used herein for which no definition is provided herein shall
have the meanings set forth in the Original Indenture.

     "Adjusted Treasury Rate" means, with respect to any Redemption Date, the
rate per annum equal to the semiannual equivalent yield to maturity of the
Comparable Treasury Issue, assuming a price for the Comparable Treasury Issue
(expressed as a percentage of its principal amount) equal to the Comparable
Treasury Price for such Redemption Date, plus 0.200%.

     "Comparable Treasury Issue" means the United States Treasury security
selected by a Quotation Agent as having a maturity comparable to the remaining
term of the Debentures to be redeemed that would be utilized, at the time of
selection and in accordance with customary financial practice, in pricing new
issues of corporate debt securities of comparable maturity to the remaining term
of such Debentures.

     "Comparable Treasury Price" means, with respect to any Redemption Date, (i)
the average of the bid and asked prices for the Comparable Treasury Issue
(expressed in each case as a percentage of its principal amount) on the third
Business Day preceding such Redemption Date, as set forth in the daily
statistical release (or any successor release) published by the Federal Reserve
Bank of New York and designated "Composite 3:30 p.m. Quotations for U.S.
Government Securities" or (ii) if such release (or any successor release) is not
published or does not contain such prices on such Business Day, (a) the average
of the Reference Treasury Dealer Quotations for such Redemption Date or (b) if
the Company obtains fewer than two such Reference Treasury Dealer Quotations,
such Reference Treasury Dealer Quotations.

     "Interest Payment Dates" means February 1 and August 1 of each year.

     "Original Issue Date" means February 1, 1999.

     "Quotation Agent" means one of the Reference Treasury Dealers appointed by
the Company and certified to the Trustee by the Company.

     "Reference Treasury Dealer" means PaineWebber Incorporated and its
successors; provided, however, that if any of the foregoing shall cease to be a
primary U.S. Government Securities dealer in New York City (a "Primary Treasury
Dealer"), the Company shall substitute therefor another Primary Treasury Dealer
and certify same to the Trustee.

     "Reference Treasury Dealer Quotations" means, with respect to each
Reference Treasury Dealer and any Redemption Date, the average, as determined by
the Company and certified to the Trustee by the Company, of the bid and asked
prices for the Comparable Treasury Issue (expressed in each case as a percentage
of its principal amount) quoted in writing to the 

                                       2
<PAGE>
 
Company by such Reference Treasury Dealer at 5:00 p.m. on the third Business Day
preceding such Redemption Date.

     "Regular Record Date" means January 15 in the case of a February 1 Interest
Payment Date and July 15 in the case of an August 1 Interest Payment Date.

     "Stated Maturity" means February 1, 2009.

     SECTION 103. Payment of Principal and Interest. The principal of the
Debentures shall be due at Stated Maturity (unless earlier redeemed). The unpaid
principal amount of the Debentures shall bear interest at the rate of 6% per
annum until paid or duly provided for. Interest shall be paid semi-annually in
arrears on each Interest Payment Date to the Person in whose name the Debentures
are registered on the Regular Record Date for such Interest Payment Date and on
Stated Maturity. Accrued interest paid on Stated Maturity shall be paid to the
Person to whom principal is paid. Any such interest that is not so punctually
paid or duly provided for will forthwith cease to be payable to the Holders on
such Regular Record Date and may either be paid to the Person or Persons in
whose name the Debentures are registered at the close of business on a Special
Record Date for the payment of such defaulted interest to be fixed by the
Trustee, notice whereof shall be given to Holders of the Debentures not less
than ten days prior to such Special Record Date.

     Payments of interest on the Debentures will include interest accrued to but
excluding the respective Interest Payment Dates. Interest payments for the
Debentures shall be computed and paid on the basis of a 360-day year of twelve
30-day months.

     Payment of the principal and interest due at the Stated Maturity or earlier
redemption of the Debentures shall be made upon surrender of the Debentures at
the office or agency of the Company in the Borough of Manhattan, City and State
of New York or at the Corporate Trust Office of the Trustee. The principal of
and interest on the Debentures shall be paid in such currency of the United
States of America as at the time of payment is legal tender for payment of
public and private debts. Payments of interest will be made at the option of the
Company, (i) by check mailed to the address of the Person entitled thereto as
such address shall appear in the Security Register or (ii) by wire transfer to
an account located in the United States maintained by the payee.

     SECTION 104. Denominations. The Debentures may be issued in denominations
of $1,000, or any integral multiple thereof.

     SECTION 105. Global Securities. The Debentures will be issued in the form
of one or more Global Securities registered in the name of the Depositary or its
nominee. Except under the limited circumstances described below, Debentures
represented by the Global Security will not be exchangeable for, and will not
otherwise be issuable as, Debentures in definitive form. The Global Securities
described above may not be transferred except by the Depositary to a nominee of
the Depositary or by a nominee of the Depositary to the Depositary or another
nominee of the Depositary or to a successor Depositary or its nominee.

     Owners of beneficial interests in such a Global Security will not be
considered the Holders thereof for any purpose under the Indenture, and no
Global Security representing a Debenture shall be exchangeable, except for
another Global Security of like denomination and tenor to be registered in the
name of the Depositary or its nominee or to a successor Depositary 

                                       3
<PAGE>
 
or its nominee. The rights of Holders of such Global Security shall be exercised
only through the Depositary.

     A Global Security shall be exchangeable for Debentures registered in the
names of persons other than the Depositary or its nominee only if (i) the
Depositary notifies the Company that it is unwilling or unable to continue as a
Depositary for such Global Security and no successor Depositary shall have been
appointed by the Company, or if at any time the Depositary ceases to be a
clearing agency registered under the Securities Exchange Act of 1934, as
amended, at a time when the Depositary is required to be so registered to act as
such Depositary and no successor Depositary shall have been appointed by the
Company, in each case within 90 days after the Company receives such notice or
becomes aware of such cessation, (ii) the Company in its sole discretion
determines that such Global Security shall be so exchangeable, or (iii) there
shall have occurred an Event of Default with respect to the Debentures.

     SECTION 106. Transfer. No service charge will be made for any transfer or
exchange of Debentures, but payment will be required of a sum sufficient to
cover any tax or other governmental charge that may be imposed in connection
therewith.

     SECTION 107. Redemption at the Company's Option. The Debentures will be
redeemable, in whole or in part, at any time at the option of the Company at a
redemption price (the "Redemption Price") equal to the greater of (i) 100% of
the principal amount of such Debentures or (ii) as determined by a Quotation
Agent, the sum of the present values of the remaining scheduled payments of
principal and interest discounted to the date of redemption (the "Redemption
Date") on a semiannual basis (assuming a 360-day year consisting of twelve 30-
day months) at the Adjusted Treasury Rate, plus, in each case, accrued but
unpaid interest to the Redemption Date.

     Notice of any redemption will be mailed at least 30 days but not more than
60 days before the Redemption Date to each Holder of the Debentures to be
redeemed. Unless the Company defaults in payment of the Redemption Price,
interest will cease to accrue on the Debentures or portions thereof called for
redemption on and after the Redemption Date.

     SECTION 108.  Other Terms.

     The Debentures will not have a sinking fund.

     Notice of redemption shall be given as provided in Section 1104 of the
Original Indenture.

     Any redemption of less than all of the Debentures shall, with respect to
the principal thereof, be divisible by $1,000.

                                       4
<PAGE>
 
                                   ARTICLE 2

                            MISCELLANEOUS PROVISIONS

     SECTION 201. Recitals by Company. The recitals in this Third Supplemental
Indenture are made by the Company only and not by the Trustee, and all of the
provisions contained in the Original Indenture in respect of the rights,
privileges, immunities, powers and duties of the Trustee shall be applicable in
respect of Debentures and of this Third Supplemental Indenture as fully and with
like effect as if set forth herein in full.

     SECTION 202. Ratification and Incorporation of Original Indenture. The
Original Indenture is in all respects ratified and confirmed, and the Original
Indenture and this Third Supplemental Indenture shall be read, taken and
construed as one and the same instrument; provided that, in the case of a
conflict between this Third Supplemental Indenture and the Original Indenture,
this Third Supplemental Indenture shall control.

     SECTION 203. Executed in Counterparts. This Third Supplemental Indenture
may be simultaneously executed in several counterparts, each of which shall be
deemed to be an original, and such counterparts shall together constitute one
and the same instrument.

     SECTION 204. Parties Interested Herein. Nothing in the Indenture expressed
or implied is intended or shall be construed to confer upon, or to give or grant
to, any person or entity, other than the Company, the Trustee, the Paying Agent
and the registered owners of the Debentures, any right, remedy or claim under or
by reason of the Indenture or any covenant, condition or stipulation hereof, and
all covenants, stipulations, promises and agreements in the Indenture contained
by and on behalf of the Company shall be for the sole and exclusive benefit of
the Company, the Trustee, the Paying Agent and the registered owners of the
Debentures.

                                       5
<PAGE>
 
          IN WITNESS WHEREOF, each party hereto has caused this Third
Supplemental Indenture to be signed in its name and behalf by its duly
authorized officers or signatories, all as of the day and year first above
written.

ATTEST:                               ONEOK, INC.



By: /s/                               By: /s/
    ------------------------              -----------------------------



ATTEST:                               CHASE BANK OF TEXAS, NATIONAL
                                      ASSOCIATION, as Trustee


By: /s/                               By: /s/                            
    ------------------------              -----------------------------

      Authorized Signatory                   Authorized Signatory

                                       6
<PAGE>
 
                                                                       EXHIBIT A


                               FORM OF DEBENTURE


                                   [Attached]

                                      A-1
<PAGE>
 
     THIS SECURITY IS A GLOBAL SECURITY WITHIN THE MEANING OF THE INDENTURE 
HEREINAFTER REFERRED TO AND IS REGISTERED IN THE NAME OF A DEPOSITORY OR A 
NOMINEE THEREOF. THIS SECURITY MAY NOT BE EXCHANGED IN WHOLE OR IN PART FOR A 
SECURITY REGISTERED, AND NO TRANSFER OF THIS SECURITY IN WHOLE OR IN PART MAY BE
REGISTERED, IN THE NAME OF ANY PERSON OTHER THAN SUCH DEPOSITORY OR ITS NOMINEE 
EXCEPT IN THE LIMITED CIRCUMSTANCES DESCRIBED IN THE INDENTURE.

     UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF THE
DEPOSITORY TRUST COMPANY, A NEW YORK CORPORATION ("DTC"), TO THE ISSUER OR ITS 
AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE, OR PAYMENT, AND ANY CERTIFICATE 
ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR IN SUCH OTHER NAME AS IS 
REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC (AND ANY PAYMENT IS MADE TO 
CEDE & CO. OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED 
REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE, OR OTHER USE HEREOF FOR VALUE OR 
OTHERWISE BY OR TO ANY PERSON IS WRONGFUL INASMUCH AS THE REGISTERED OWNER 
HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

================================================================================
No. __

                                 $100,000,000
                                  ONEOK, INC.
                      6% DEBENTURES DUE FEBRUARY 1, 2009

                               CUSIP: 682680AC7

     ONEOK, Inc., an Oklahoma corporation (herein called "Company," which term 
includes any successor corporation under the Indenture referred to herein), for 
value received, hereby promises to pay to:

     __________

or registered assigns, the principal sum of

     *___________________________*

on February 1, 2009 and to pay interest on such principal sum at the rate of six
percent (6%) per annum.

     The Company will pay interest from February 1, 1999, semi-annually on 
February 1 and August 1 of each year, and on the date of maturity, commencing 
August 1, 1999 (each such date an "Interest Payment Date"), until the principal 
hereof is otherwise paid or duly provided for. The interest so payable, and 
punctually paid or duly provided for, on any Interest Payment Date will, as 
provided in the Indenture (as defined below), be paid to the holder of this 
Debenture (the "Holder") of record at the close of business on the regular 
record date (the "Regular Record 


<PAGE>

Date") for such Interest Payment Date, which, except for interest payable on
February 1, 2009, shall be January 15 (in the case of the February 1 Interest
Payment Date) or July 15 (in the case of the August 1 Interest Payment Date),
whether or not a Business Day. Interest will be computed on the basis of a 360-
day year of twelve 30-day months.

     Any such interest not so punctually paid or duly provided for shall
forthwith cease to be payable to the Holder on such Regular Record Date by
virtue of his having been such Holder, and may be paid to the Holder of record
of this Debenture at the close of business on a special record date (the
"Special Record Date") fixed by the Company for the payment of such defaulted
interest, notice whereof shall be given to Holders not less than 10 days prior
to such Special Record Date, all as more fully provided in the Indenture.

     Payment of the principal of this Debenture and the interest thereof will be
made at the office or agency of the Company in the Borough of Manhattan, City 
and State of New York or at the Corporate Trust Office of the Trustee in such 
currency of the United States of America as at the time of payment is legal 
tender for payment of public and private debts.


<PAGE>
 
                                  ONEOK, INC.
                      6% Debentures Due February 1, 2009

     This Debenture is one of a duly authorized issue of debt securities of the 
Company (herein called the "Securities"), issuable in one or more series, issued
and to be issued under and pursuant to an Indenture dated as of September 24, 
1998, as amended and supplemented by that certain Third Supplemental Indenture 
(the "Indenture"), duly executed and delivered by the Company to Chase Bank of 
Texas, National Association, as trustee (the "Trustee," which term includes any 
successor trustee under the Indenture) and is one of a series unlimited in 
aggregate principal amount and designated as 6% Debentures Due February 1, 2009 
(the "Debentures"). Reference is hereby made to the Indenture for a description 
of the rights, limitations of rights, obligations, duties and immunities 
thereunder of the Trustee, the Company and the holders of Securities (including 
Holders of the Debentures).

     The Debentures are subject to defeasance at the option of the Company as 
provided in the Indenture.

     As long as this Debenture is represented in global form (the "Global 
Security") registered in the name of the Depository or its nominee, except as 
provided in the Indenture and subject to certain limitations therein set forth, 
no Global Security shall be exchangeable or transferable.

     If an Event of Default (as defined in the Indenture) with respect to the 
Debentures shall occur and be continuing, the principal plus any accrued 
interest may be declared due and payable in the manner and with the effect and 
subject to the conditions provided in the Indenture.

     The Indenture permits the amendment thereof and the modification of the 
rights and obligations of the Company and the rights of the Holders under the 
Indenture at any time by the Company and the Trustee with the consent of the 
Holders of a majority in aggregate principal amount of the Securities then 
Outstanding (as defined in the Indenture) of all series which are affected by 
such amendment or modification, except that certain amendments which do not 
adversely affect the rights of any holder of the Securities may be made without 
the approval of holders of the Securities. No amendment or modification may, 
among other things, change the Stated Maturity of any Security, reduce the 
principal amount thereof, reduce the rate or change the time of payment of any 
interest thereon, or reduce the aforesaid majority in aggregate principal amount
of Securities of any series, consent of the holders of which is required for any
such amendment or modification, without the consent of each Securityholder 
affected.

     Notwithstanding any provision in the Indenture or any provision of this 
Debenture, the Holder of this Debenture shall have the right, which is absolute 
and unconditional, to receive payment of the principal of (and premium, if any) 
and interest on this Debenture at the times, places and rate, and in the
currency herein prescribed.

     The Debentures may be redeemed, in whole or in part, at any time at the 
option of the Company at a Redemption Price equal to the greater of (i) 100% of 
the principal amount of such Debentures or (ii) as determined by a Quotation 
Agent, the sum of the present values of the remaining scheduled payments of 
principal and interest discounted to the date of redemption on a semiannual 
basis (assuming a 360-day consisting of twelve 30-day months) at the Adjusted 
Treasury Rate, plus, in each case, accrued but unpaid interest to the Redemption
Date.



<PAGE>
 
     In the event of redemption of the Debenture in part only, a new Debenture 
or Debentures of this series for the unredeemed portion hereof will be issued in
the name of the Holder upon surrender hereof or otherwise reduced in accordance 
with the provisions of the Indenture. The Debentures will not have a sinking 
fund.

     THIS DEBENTURE SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE 
LAWS OF THE STATE OF NEW YORK, WITHOUT REGARD TO PRINCIPLES OF CONFLICTS OF 
LAWS.

     All terms used in this Debenture but not defined herein have the meanings 
assigned to them in the Indenture.

     Unless the certificate of authentication hereon has been executed by or on 
behalf of the Trustee by manual signature, this Debenture shall not be entitled 
to any benefit under the Indenture or be valid or obligatory for any purpose.



<PAGE>
 
     IN WITNESS WHEREOF, the Company has caused this Debenture to be duly 
executed.

                             ONEOK, INC.


                             By: __________________________________________
                                   Name: 
                                   Title: 



                             Attested: ____________________________________
                                            Name: 
                                            Title: 


TRUSTEE'S CERTIFICATE OF AUTHENTICATION

Dated: 

This is one of the Debentures referred to in the within-
mentioned Indenture.

                      CHASE BANK OF TEXAS, NATIONAL ASSOCIATION
                       as Trustee [or Successor Trustee]


                      By: ____________________________
                            Authorized Signatory
<PAGE>
 
              SCHEDULE OF EXCHANGES OF INTEREST IN THE DEBENTURE

     The following exchanges of interests in this Debenture have been made:

<TABLE> 
<CAPTION> 
                                                                Principal Amount of
                   Amount of decrease   Amount of increase    this Debenture following     Signature of authorized
Date of Exchange    in this Debenture   in this Debenture    such decrease or (increase)    signatory of Trustee
<S>                <C>                  <C>                  <C>                           <C> 
</TABLE> 
<PAGE>
 
                                ASSIGNMENT FORM

               To assign this Debenture, fill in the form below:
                 I or we assign and transfer this Debenture to

                             ---------------------

                             ---------------------

                  Insert assignee's soc. sec. or tax I.D. no.

                             ---------------------


________________________________________________________________________________
             (Print or type assignee's name, address and zip code)


________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
and all rights thereunder and irrevocably appoint  _____________________________
________________________________________________________________________________
agent to transfer this Debenture on the books of the Company. The agent may 
substitute another to act for him.
________________________________________________________________________________

Dated: ___________________________           ___________________________________

                                  ______________________________________________

     THE SIGNATURE TO THIS ASSIGNMENT MUST CORRESPOND WITH THE NAME AS IT 
APPEARS ON THE FIRST PAGE OF THE DEBENTURE.

     THE SIGNATURE MUST BE GUARANTEED BY AN "ELIGIBLE GUARANTOR INSTITUTION" 
THAT IS A MEMBER OR PARTICIPANT IN A "SIGNATURE GUARANTEE PROGRAM" (E.G., THE 
SECURITIES TRANSFER AGENTS MEDALLION PROGRAM, THE STOCK EXCHANGE MEDALLION 
PROGRAM OR THE NEW YORK STOCK EXCHANGE, INC. MEDALLION PROGRAM).
<PAGE>
 
                                                                       EXHIBIT B


                         CERTIFICATE OF AUTHENTICATION

     This is one of the Debentures referred to in the within-mentioned
Indenture.

                                         CHASE BANK OF TEXAS, NATIONAL
                                          ASSOCIATION, as Trustee
                                          [or Successor Trustee]


     Dated: ______________________       By: ___________________________
                                                Authorized Signatory

                                      B-1


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